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Old 07-28-2013, 11:05 AM   #1 (permalink)
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Streamlining. More important, the front or the back?

I'm hoping someone with Solidworks flow simulation can run a simple experiment for us. Create a rough tear drop. Say 2 feet max diameter and 10 feet long. Round in the front and straight sides tapering back will work if it makes it easier to create the shape without the complex curves. Compare the performance of that with the same shape that has been cut off flat on the front at 80% of the radius. And to a shape with the round front but a straight cylinder all the way back to a flat back. And finally, to the original best shape that is truncated at 60% of the length with a flat back.

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Old 07-28-2013, 07:05 PM   #2 (permalink)
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I think it's been pretty well proven that, all other things equal, there are more gains at the rear. Suppose another bit can't hurt.
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Old 07-28-2013, 07:40 PM   #3 (permalink)
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Why? To what point?

It sounds like you want to compare a bullet, an 'ice cream cone' and a truncated teardrop, all at 5 to 1 fineness ratio. And a hollow-point ice cream cone.

That fineness ratio is long for a full body (2.5-4:1) but short for a blister on a larger body(10:1).

You could generate low resolution results in Solidworks. Those results *may* translate to similar comparisons in the real world, but what do you do with that information?

There are published results for similar forms.
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Old 07-28-2013, 08:40 PM   #4 (permalink)
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Quote:
Originally Posted by freebeard View Post
Why? To what point?
Why not? So I know. Links?
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Old 07-29-2013, 03:49 PM   #5 (permalink)
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performance

Quote:
Originally Posted by sendler View Post
I'm hoping someone with Solidworks flow simulation can run a simple experiment for us. Create a rough tear drop. Say 2 feet max diameter and 10 feet long. Round in the front and straight sides tapering back will work if it makes it easier to create the shape without the complex curves. Compare the performance of that with the same shape that has been cut off flat on the front at 80% of the radius. And to a shape with the round front but a straight cylinder all the way back to a flat back. And finally, to the original best shape that is truncated at 60% of the length with a flat back.
The 2' by 10' ice cream cone will have a free-flight Cd of around 0.086.
In ground effect it will be Cd 0.172.With wheels,maybe Cd 0.222,depending on how far they project outside the body.
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I'd like a drawing about the 80% nose to help think about that.
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The body with convex hemi nose and cylindrical section is Cd 0.201.
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For the truncated ice cream cone I'll have to get home and check my references.
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Old 07-29-2013, 04:01 PM   #6 (permalink)
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Cross-posted. About the 80% nose—the front can be flat with a radius IIRC around 4% of the gross width.
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Old 07-29-2013, 04:53 PM   #7 (permalink)
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60% truncation

Thanks freebeard!
From the fuselage chart it looks like the 60% truncation will cause a 28+% drag increase,so we'd be looking at Cd 0.284 with that tail chop.
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As to the nose truncation with leading edge radii,according to Hucho,that minimum would get you your flow attachment,and further rounding would net zero additional drag reduction.
************* The caveat is with 'crosswinds'. In a yawed flow condition (which is the statistical norm for N.America) the full,bulbous nose would have a distinct drag advantage.(think Oscar Mayer Weinermobile)
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Old 07-29-2013, 06:56 PM   #8 (permalink)
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Quote:
Originally Posted by freebeard View Post
"Knowledge Is Good" Emil Faber
Thanks for the replies. My question stems from discussions of whether streamlining the front or the back of a motorcycle will make the most improvement if you were only to do one or the other. And, is it worth the trouble to run the tail at full length or will a Kamm truncation to 60% of the ideal length be only a marginal loss. The hemisphere in front of a cone was just my way of over-simplifying the shape so it would be still useful for analysis but also very easy to draw.
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The fuselage example above is much too long to for our discussion of Kamm truncation since most of the length along the sides is nearly straight. By the time you get to 60% of the length, you have made a long round nosed bullet with almost no angle remaining and haven't satisfied the other rule of thumb for Kamm which is to end with a width that is no more than half of the max width of the airfoil. The angle of my tail I have started to build is about 10.5* per side, 18 inches at the front, truncated to 8 inches at the back by 28 inches long.
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That it only takes 4% of the width for the radius of a round over on the front of my tail to harbor attachment is good news as that is only .75 inches which should be very easy to exceed with a pool noodle cut in half.
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Old 07-29-2013, 07:08 PM   #9 (permalink)
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This drawing would be a good example of the tail I am building and what effects a truncation will have Though I am having trouble correlating the ratio that is displayed into our concept.
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http://ecomodder.com/forum/member-ae...ng-effect.html
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Old 07-29-2013, 08:03 PM   #10 (permalink)
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Can't say for sure. Yes it is our common knowledge that the big gains are at the rear however you are talking about a single track vehicle which- largely due to it's proportions- has less ground effect interference. In addition, I read posts from a very knowledgeable guy on here once that said he found good results with a front fairing and negligible results with a tail on bicycles (I would not have expected that).

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